This work proves for the first time the bioelectrochemical production of butyrate from CO2 as a sole carbon source. The highest concentration of butyrate achieved was 20.2 mMC, with a maximum butyrate production rate of 1.82 mMC d(-1). The electrochemical characterisation demonstrated that the CO2 reduction to butyrate was hydrogen driven. Production of ethanol and butanol was also observed opening up the potential for biofuel production.

Microbial electrosynthesis is potentially a sustainable biotechnology for the conversion of the greenhouse gas CO2 into carboxylic acids, thus far mostly limited to acetic acid (C2). Despite the environmental benefits of recycling CO2 emissions to counter global warming, bioelectrochemical production of acetate is not very attractive from an economic point of view. Conversely, carboxylates and corresponding alcohols with longer C content not only have a higher economical value as compared to acetate, but they are also relevant platform chemicals and fuels used on a diverse array of industrial applications. Here, we report on a specific mixed reactor microbiome capable of producing a mixture of C4 and C6 carboxylic acids (isobutyric, n-butyric, and n-caproic acids) and their corresponding alcohols (isobutanol, n-butanol, and n-hexanol) using CO2 as the sole carbon source and reducing power provided by an electrode. Metagenomic analysis supports the hypothesis of a sequential carbon chain elongation process comprised of acetogenesis, solventogenesis, and reverse β-oxidation, and that isobutyric acid is derived from the isomerization of n-butyric acid.

The conversion of electrical current into methane (electromethanogenesis) by microbes represents one of the most promising applications of bioelectrochemical systems (BES). Electromethanogenesis provides a novel approach to waste treatment, carbon dioxide fixation and renewable energy storage into a chemically stable compound, such as methane. This has become an important area of research since it was first described, attracting different research groups worldwide. Basics of the process such as microorganisms involved and main reactions are now much better understood, and recent advances in BES configuration and electrode materials in lab-scale enhance the interest in this technology. However, there are still some gaps that need to be filled to move towards its application. Side reactions or scaling-up issues are clearly among the main challenges that need to be overcome to its further development. This review summarizes the recent advances made in the field of electromethanogenesis to address the main future challenges and opportunities of this novel process. In addition, the present fundamental knowledge is critically reviewed and some insights are provided to identify potential niche applications and help researchers to overcome current technological boundaries.

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